Thermo-chemical surface treatments of steel by means of carbon or nitrogen carrying gases are well-known processes, called case-hardening or carburization or nitriding. Nitro-carburization is a process in which a gas caring both carbon and nitrogen is used. These processes are traditionally applied to improve the hardness and wear resistance of iron and low alloyed steel articles. The steel article is exposed to a carbon and/or nitrogen carrying gas at an elevated temperature for a period of time, whereby the gas decomposes and carbon and/or nitrogen atoms diffuse through the steel surface into the steel material. The outermost material close to the surface is transformed into a layer with improved hardness, and the thickness of this layer depends on the treatment temperature and the treatment time.
Stainless steel has excellent corrosion properties, but is relatively soft and has poor wear resistance, especially against adhesive wear. Therefore, there is a need of improving the surface properties for stainless steel. Gas carburization, nitriding and nitro-carburizing of stainless steel involve some difficulties, as the passive layer, causing the good corrosion properties, acts as a barrier layer preventing carbon and/or nitrogen atoms from diffusing through the surface. Also the elevated temperatures of the treatments promote the formation of chromium carbides or chromium nitrides. The formation of chromium carbides and/or chromium nitrides reduces the free chromium content in the material whereby the corrosion properties are deteriorated.
Several methods of case-hardening stainless steel have been proposed by which these drawbacks are minimized or reduced.
It is known that a pre-treatment in a halogen-containing atmosphere provides an effective activation of the surface.
EP 0588458 discloses a method applying fluorine as an active component in a gas pre-treatment, where the passive layer of the stainless steel surface is transformed into a fluorine-containing surface layer, which is permeable for carbon and nitrogen atoms.
Plasma-assisted thermo-chemical treatment and ion implantation have also been proposed. In this case the passive layer of the stainless steel is removed by sputtering, which is an integrated part of the process.
EP 0248431 B1 discloses a method for electroplating an austenitic stainless steel article with iron prior to gas nitriding. The nitrogen atoms can diffuse through the iron layer and into the austenitic stainless steel. After gas nitriding, the iron layer is removed, and a hardened surface is obtained. In the only example of this patent, the process is carried out at 575° C. for 2 hours. At this temperature, chromium nitrides are formed, whereby the corrosion properties are deteriorated.
EP 1095170 discloses a carburization process in which an article of stainless steel is electroplated with an iron layer prior to carburization. A passive layer is avoided, and carburization can be carried out at a relatively low temperature without the formation of carbides.
NL 1003455 discloses a process in which an article of iron or a low alloyed steel is plated with a layer of e.g. nickel before gas nitriding. Nickel protects the iron from oxidation and serves as a catalytic surface for the decomposition of the NH3 gas. The process can be carried out at temperatures below 400° C., and the purpose is to obtain a pore-free iron nitride layer.